A. Field of the Invention
This invention relates generally to medical diagnostic devices. More particularly, the present invention provides an apparatus and a method for measuring the tension of carpal ligaments under applied translation forces which in turn will provide indicators of future or existing carpal tunnel syndrome.
B. Theory of the Invention
Carpal tunnel syndrome is a condition in which the median nerve is compressed by the surrounding contents of the median nerve canal. This condition is believed to be caused by a biomechanical ligament imbalance in the volar carpal ligaments, which is in turn related to the increased ratio of power produced by the flexor muscles over extensor muscles as they interact with the hand.
The flexor muscle tendons of the forearm acting on the wrist, fingers and thumb exert a collective static force power many times greater, volarly, than the extensor muscle tendons acting to stabilize the same members of the wrist and hand dorsally. The ratio of these opposing forces is normally four to one. However, work demands often increase this ratio through hypertrophy of the flexor muscle tendon units by intensity and duration of tasks requiring dominantly finger, thumb, and wrist function.
The effect of the volar flexor forces, acting upon the palmer transverse carpal ligament (PCTL) as a pulley, attenuate the PCTL and apply forces anteriorly and medially. This places traction forces to the ligament ends of the carpus. Each night, while the muscles are at rest, the intercarpal segments restore their normal position grossly; however, some minute anteriomedial deformity occurs, and slack of the PCTL is concurrently taken up by contractile forces of the ligament(s). Numerous cycles of force followed by rest develop an established deforming characteristic narrowing the horeshoe ends of the carpal tunnel, which are held in position by a thickening PTCL and other volar carpal ligaments. Simultaneously, the PTCL acting as a pulley concentrates the load of the finger and thumb function initiating a volar glide, that is, movement as a unit in a volar direction, of the carpal metacarpal complex. This volar glide of the carpal metacarpal complex attenuates the predisposed thin dorsal carpal ligaments (DCL) originating from the distal radial ulna (DRU). The volar carpal ligaments collectively, non stressed, begin to contract, encouraging the anteriomedial collapse of the intercarpal spaces.
Flexor power exerts applied forces to the moment arm of a joint whose motion occurs in the direction of the muscle origin regulated by an interplay of antagonists, pulleys and joint alignment. A variation of one or more serves to simplify convergence towards a direct line to the point of origin and shorten the distance between. Either the force is expressed in efficiency or a decreasing biomechanical advantage occurs by a volar shift of the axis of the proximal carpal row. This may account for the propensity of carpal tunnel patients to develop odd compensatory behaviors like flexing the wrist during power grasping, conceivably to account for the change in position of the more volar placed PTCL. Carpal tunnel volume is reduced and any other predispositon will hasten onset of the painful condition known as carpal tunnel syndrome.
Thus, the resistance of the PCTL and related volar ligaments to movement returning the carpal metacarpal complex to a neutral position, i.e. dorsal glide, should be indicative of the severity of the condition of carpal tunnel syndrome or the propensity of the subject to incur the condition.
C. Description of the Related Art
There are numerous systems in the prior art for measuring forces exerted by the forearm, wrist, and fingers. One such system is described in U.S. Pat. No. 5,163,443, by Fry-Welch et al. (Nov. 17, 1992). It describes a system for measuring volitional forces applied by the hand, wrist, and forearm of a living person. A method for correlating the forces exerted in a plurality of directions so that they can be measured by a single force transducer is described. The system measures muscle force exerted by the wrist and forearm motions and correlates these to the forces required to do specific work tasks which contribute to Carpal Tunnel Syndrome. However, this system does not include or teach a mechanism for immobilizing the wrist in relation to the forearm so that measurements relative to these two body components can be made. Also, the invention described by Fry-Welch measures volitional muscular forces; there is no provision for applying an outside force, measuring the resistance of passive components, such as the ligaments, and measuring positional joint alignment in an anterior posterior plane.
An article by Jobbins, Bird, and Wright, entitled "A Joint Hyperextensometer for the Quantification of Joint Laxity", Engineering in Medicine, Vol 8 No. 2, 1979, describes a device for measuring the magnitude of joint laxity of the fifth metacarpo-phalangeal (MCP) joint. The device consists of immobilizing the joint so that the angle of displacement of the joint from the zero baseline can be measured. A mechanical slipping clutch exerts a predetermined force on the joint. However, these measurements are applicable to the areas of rheumatic arthritis and osteoarthrosis, and not carpal tunnel syndrome. They are taken with relation to the MCP joint and not the carpal joint. The measurement taken is one of force versus angle of movement in a single plane; joint alignment, which involves the normal displacement between two different planes, is not discussed. They conclude by stating that, in the test population, laxity at the MCP joint of the index finger reflects joint laxity elsewhere in the body; this observation would not hold true for such conditions as carpal tunnel syndrome which are caused by repetitive and cumulative motions by a particular joint of the body.
Marcus et al., U.S. Pat. No. 4,986,280, teaches a mechanism for sensing the relative angular orientation of two relatively movable joint segments of a living body joined together at a joint, focusing primarily on the fingers and thumb. However, it does not measure the orientation of the planes of the hand and forearm with one another or specifically address the carpal joint.
The previous methods for measuring the conditions which precipitate carpal tunnel syndrome have the following disadvantages:
1. They concentrate on measuring muscular force which is voluntarily exerted in a repetitive fashion rather than the resistive forces of the ligaments and joints. PA1 2. They do not predict the onset or the degree of damage or malalignment leading to carpal tunnel syndrome. PA1 3. They do not measure forces exerted on the carpal joint. PA1 4. They do not immobilize the forearm and wrist in such a way that one can measure the orientation between these two components in terms of the displacement between the planes of the wrist and forearm.
For the foregoing reasons, a new method and apparatus is needed for measuring the conditions which lead to carpal tunnel syndrome so as to predict the onset of the condition, isolate the anatomical areas which are pertinent to the condition, and which is non invasive.